Recently, the consumption of liquefied gas, such as liquefied natural gas (LNG) or liquefied petroleum gas (LPG), has been rapidly increasing throughout the world. Liquefied gas is transported in a gas state through onshore or offshore gas pipelines, or is transported to a remote consumption place while being stored in a liquefied state inside a liquefied gas carrier. Liquefied gas, such as LNG or LPG, is obtained by cooling natural gas or petroleum gas to a cryogenic temperature (in the case of LNG, about −163° C.). Since the volume of liquefied gas is considerably reduced as compared to a gas state, liquefied gas is very suitable for a long-distance marine transportation.
A liquefied gas carrier such as an LNG carrier is designed to load liquefied gas, sail across the sea, and unload the liquefied gas at an onshore consumption place. To this end, the liquefied gas carrier includes a storage tank (also called “cargo tank”) that can withstand a cryogenic temperature of liquefied gas.
Examples of a marine structure provided with a cargo tank capable of storing cryogenic liquefied gas may include vessels such as a liquefied gas carrier and an LNG Regasification Vessel (LNG RV), or structures such as an LNG Floating Storage and Regasification Unit (LNG FSRU) and an LNG Floating, Production, Storage and Off-loading (LNG FPSO), and a Barge Mounted Power Plant (BMPP).
The LNG RV is a self-propelled, floatable liquefied gas carrier equipped with an LNG regasification facility, and the LNG FSRU is a marine structure that stores LNG unloaded from an LNG carrier on the sea far away from the land and, if necessary, supplies the LNG to an onshore consumption place by gasifying the LNG. The LNG FPSO is a marine structure that refines extracted LNG on the sea, stores the LNG in a storage tank after direct liquefaction, and, if necessary, transships the LNG to an LNG carrier. The BMPP is a structure that is equipped with a power generation facility to produce electricity on the sea.
The term “vessel” as used herein is a concept including a liquefied gas carrier such as an LNG carrier, an LNG RV, and structures such as an LNG FPSO, an LNG FSRU, and a BMPP.
Since the liquefaction temperature of natural gas is a cryogenic temperature of −163° C. at ambient pressure, LNG is likely to be vaporized even when the temperature of LNG is slightly higher than −163° C. at ambient pressure. In the case of a conventional LNG carrier, even though an LNG cargo tank is thermally insulated, external heat is continuously transferred to LNG. Therefore, during the transportation of LNG by the LNG carrier, LNG is continuously vaporized within the LNG cargo tank and boil-off gas (hereinafter, referred to as BOG) is generated within the LNG cargo tank.
The generated natural gas may increase the inside pressure of the cargo tank and accelerate the flow of the natural gas due to the rocking of the vessel, causing structural problems. Therefore, it is necessary to suppress the generation of BOG.
Conventionally, in order to suppress the generation of BOG within the cargo tank of the liquefied gas carrier, a method of discharging the BOG from the cargo tank and burning the BOG, a method of discharging the BOG from the cargo tank, reliquefying the BOG through a reliquefaction apparatus, and returning the BOG to the cargo tank, a method of using the BOG as fuel for a vessel's propulsion engine, and a method of suppressing the generation of BOG by maintaining an inside pressure of a cargo tank at a high level have been used solely or in combination.
In the case of a conventional vessel equipped with a BOG reliquefaction apparatus, BOG inside a cargo tank is discharged from the cargo tank and then reliquefied through a reliquefaction apparatus in order to maintain a pressure of the cargo tank at an appropriate level. In this case, the discharged BOG is reliquefied through heat exchange with a refrigerant (for example, nitrogen, mixed refrigerant, or the like) cooled to a cryogenic temperature in the reliquefaction apparatus including a refrigeration cycle, and the reliquefied BOG is returned to the cargo tank.
In the case of a conventional LNG carrier equipped with a DFDE propulsion system, BOG is consumed in such a manner that it is supplied as fuel to the DFDE after treating BOG by only a BOG compressor and heating, without installing the reliquefaction facility. Therefore, when an amount of fuel necessary for an engine is smaller than a generation amount of BOG, there is a problem that BOG is burnt in a gas combustion unit (GCU) or is vented to atmosphere.
Even though a conventional LNG carrier equipped with a reliquefaction facility and a low-speed diesel engine can treat BOG through the reliquefaction facility, the control of the entire system is complicated due to the operation complexity of the reliquefaction facility using nitrogen gas, and a considerable amount of power is consumed.
Consequently, there is a need for continuous research and development of systems and methods for efficiently treating liquefied gas, including BOG generated naturally from the cargo tank.